1. Field of the Invention
This invention relates to the manufacture of water-absorbent clothing or felt used on papermaking machines. More specifically, it involves a method by which a uniform layer of polymeric resin particles can be applied to the surface of a textile base fabric and fused to provide a porous, elastic surface.
2. Description of the Prior Art
One of the key components of a modern papermaking machine is the water-absorbent machine clothing or felt. These so-called felts take the form of endless belts of considerable size, and are custom-made according to specifications, supplied by the papermaker, which include the dimensions of the belt required to clothe a particular machine position and the kind of paper the machine produces.
The felts used in the press section of a papermaking machine concern us here. There, the felts serve at least two important purposes, as they both support the wet fibrous sheet as it is being transformed into the finished paper product and absorb water from that sheet in great quantities.
This can be better understood if one views the press section in terms of its relationship to the other sections of the papermaking machine. The first section, immediately before the press section, is the forming section, where the wet fibrous sheet is formed by spraying an aqueous suspension of wood pulp fiber on a fine screen or wire. A great deal of water will drain out of the sheet in this stage, but water will still account for a major proportion of its weight as it reaches the end of the forming section. As a result, the sheet has little structural integrity at this point, and will require support if it is not to break and cause machine operating problems.
From the forming section, the sheet proceeds to the press section. There, the press felts provide the necessary support to the sheet as it makes its passage through the presses, where additional large quantities of water are squeezed out. Many papermachine press sections incorporate two or more such felts, and, quite often, the wet sheet will be carried or sandwiched between two felts as it makes its journey through the section. In any case, it is in the press nip, the narrow region between the press rolls where compression occurs, that the felt carries out the function of absorbing the water squeezed out of the wet sheet.
It is important, from the standpoint of economics, that the felt be capable of removing as much water as possible during the transit of the sheet through the press section. This is so because, after leaving that section, the sheet enters the dryer section, where whatever water that remains is evaporated through the use of heated rolls. Costs associated with this heating will be reduced when optimum quantities of water are absorbed by the felt in the press section, as less water will have to be removed in the dryer section.
Up until fairly recently, wool was the basic raw material used in the production of press felts. As the term "felt" suggests, one of the final steps in the production of these machine belts was that of felting, in which the woven woolen fabric was wet and subjected to rubbing in order to produce a belt having a smooth surface. Today, however, with the advent of a great variety of synthetics, wool has largely fallen out of use.
Contemporary synthetics carry the advantages of greater strength, durability, and resistance to chemical and bacterial attack when compared to woolen felt. The term "felt", strictly speaking, is no longer applicable to these fabrics as they cannot be felted in the literal sense. Instead, alternate manufacturing steps are taken during their production to give them surface characteristics and finish similar to those of true felt. For example, the so-called batt-on-base felts, currently in wide use and considered the standard of the industry, consist of a woven fabric base with a batt surface attached by needling and have surface characteristics similar to those of woolen felt. In addition, a wide variety of other constuctions are available, including non-woven press felts. Yet, despite the complete absence of wool and traditional felting processes during production, they are still commonly, if not universally, referred to as "felts" in the papermaking industry.
Press felt are characterised by such factors as fiber variety, weave type, permeability, and surface characteristics. The choice of felt to be used on any given machine is governed by the machine design and operating parameters, the grade of paper being produced and the desired surface finish.
As noted above, there are alternatives available for use instead of the popular batt-on-base press felts. For example, U.S. Pat. No. 4,571,359 entitled "Paperpermaker's Wet-Press Felt and Method of Manufacture", disclosed a novel papermaker's felt composed of a textile base fabric having a surface layer of polymeric resin particles fused together to provide a porous, elastic surface. In practice, however, it has proven difficult to apply the resin particles in a manner that will leave them uniformly and evenly distributed on the surface of the base fabric. The present invention is addressed to solving that problem.